The invention relates to an improved tunable optical filter and modulator.
Thin film bandpass filters are key components in wavelength-division multiplexing (WDM) systems, in particular density wavelength-division multiplexing (DWDM) systems. A typical thin film bandpass filter comprise a series of dielectric resonance cavities, with quarter wave thick layers of lower index material separating them. These filters are also sometimes referred to as dielectric multilayer interference filters. Many improvements in the performance of these filters, such as improved wavelength response and polarization independency, have been made. U.S. Pat. No. 6,018,421 which issued to Cushing on Jan. 25, 2000 provides a summary of known bandpass filters of this type along with disclosure of an improvement whereby the transition slopes of the filter are improved as well as removal of the ripple effect generally associated with these filters.
For WDM applications and, especially for DWDM applications, tunable optical filters are becoming useful. It is known to use thin film filters as tunable optical filters. Typically, such a tunable optical filter comprises a thin film filter mounted on a rotatable surface such that the angle of incidence of the incoming light on the filter can be altered by rotation of the surface. The change in the incident angle alters the pass band of the filter. However, these tunable filters have slow response time due to the mechanical rotation of the thin film filter. Further, because of the reflective angles created by the multilayers within the filter, the filter response becomes polarization dependent as the incident angle is altered. U.S. Pat. No. 5,481,402 which issued to Cheng et al on Jan. 2, 1996 provides an improved tunable optical filter of this nature wherein the light is passed through the filter, through a quarter wave plate and onto a reflective surface which reflects the light back through the quarter wave plate and through the filter a second time. Although this technique does improve the polarization dependency of the filter, the filter remains polarization dependent to a limited extent. Further, the tunable optical filter disclosed has the same slow response time as discussed above due to the mechanical tuning of the filter.
Other types of tunable optical filters known in the art are acoustic-optical tunable filters (AOTF) and liquid crystal tunable filters (LCTF). The AOTFs are undesirable for many applications because of the high insertion losses and because the passband of these filters is not sufficiently narrow. An example of a LCTF know in the art is disclosed in U.S. Pat. No. 5,321,539 which issued to Hirahayashi et al on Jun. 14, 1994. LCTFs are undesirable in certain circumstances due to slow response time and the polarization dependency of these filters.
Further, switches and modulators have become key components in modern optical communication systems. External modulation has become necessary for high bit rate systems, such as OC-192 . Mach-Zehnder interference modulators are well known in the art and commonly used as optical modulators or switches. These interference modulators consist of a single input waveguide, an input branching region for splitting the input light into two substantially equal portions into two branch waveguides and an output waveguide. By effecting a phase shift in one branch of the waveguide relative to the other, the output light power can be altered between zero and the input light power level, depending on the phase shift. The phase shifts are generally effected by means of electrodes disposed on the substrate of one or both of the branch waveguides. Although these modulators are effective, the costs of manufacturing these devices are high and the devices have high insertion losses, are polarization dependent and have poor thermal properties.
The invention is based on a conventional thin film optical filter. The thickness of the thin film optical filter determines the optical properties of the filter, specifically the pass band. The thin film optical filter is arranged within a hole extending through a layer of piezoelectric material such that the periphery of the thin film optical filter is secured to the periphery of the hole. Electrodes are provided on opposite surfaces of the piezoelectric layer such that different voltage levels can be applied. The resulting deformation of the piezoelectric layer causes a difference in the thickness of the thin film optical filter thereby permitting tuning of the optical filter.
Instead of a single layer of piezoelectric material a multi-layered arrangement of many thin layers of piezoelectric material could be used. In this case the layers are sandwiched between transparent substrates and electrodes.
Although the main embodiment suggested involves an optical filter completely surrounded by the piezoelectric material, in an alternative embodiment a narrow strip of thin film optical filter is provided with piezoelectric material only along the two longer sides.
The invention also provides a method of making the novel tunable optical filter.
The improved tunable filter can also be used as a modulator or a switch. As an essentially fixed wavelength filter, the invention provides a way of fine tuning the passband of conventional thin film filters such that variations in the filter due to changes in the surrounding environment, such as temperature, can be compensated for. As a tunable filter, although the dynamic range of a filter according to the invention is less then many conventional tunable filters, the tunable filter of the invention has a quicker response time and no polarization dependency. Further, when used in a DWDM system, it allows the density of the DWDM system to be significantly increased. The filter may also be used to replace several filters in multichannel signal devices as it can be switched to selectively transmit different wavelength signals corresponding to different channels. As a modulator, the invention is inexpensive compared to conventional modulators and provides improved performance characteristics.